Enhancement of thermoelectric properties by energy filtering: Theoretical potential and experimental reality in nanostructured ZnSb

Abstract

Energy filtering has been suggested by many authors as a means to improve thermoelectric properties. The idea is to filter away low-energy charge carriers in order to increase Seebeck coefficient without compromising electronic conductivity. This concept was investigated in the present paper for a specific material (ZnSb) by a combination of first-principles atomic-scale calculations, Boltzmann transport theory, and experimental studies of the same system. The potential of filtering in this material was first quantified, and it was as an example found that the power factor could be enhanced by an order of magnitude when the filter barrier height was 0.5 eV. Measured values of the Hall carrier concentration in bulk ZnSb were then used to calibrate the transport calculations, and nanostructured ZnSb with average grain size around 70 nm was processed to achieve filtering as suggested previously in the literature. Various scattering mechanisms were employed in the transport calculations and compared with the measured transport properties in nanostructured ZnSb as a function of temperature. Reasonable correspondence between theory and experiment could be achieved when a combination of constant lifetime scattering and energy filtering with a 0.25 eV barrier was employed. However, the difference between bulk and nanostructured samples was not sufficient to justify the introduction of an energy filtering mechanism. The reasons for this and possibilities to achieve filtering were discussed in the paper.

Review of Research on the Thermoelectric Material ZnSb

“…(c) 3×1×3 cell shows the Zn 2 Sb 2-ring network. Each ring has six nearest neighbours, connected by bonds ix [1][2][3][4][5][6] . (d) The angles between Sb-Zn-Sb are similar to that of the standard tetrahedron, 109.5°.…”

“…(d) The angles between Sb-Zn-Sb are similar to that of the standard tetrahedron, 109.5°. The bonds i-vi are covalent bonds for building up the tetrahedron, while vii and viii are the bonds to form Zn 2 Sb 2 -ring; bonds ix [1][2][3][4][5][6] in (c) are the dimers, that connect the Zn 2 Sb 2 -rings.…”

“…The band structure of ZnSb has been calculated by ab-initio methods by many groups in recent years [2,26,31,[36][37][38][39][40][41][42]. The calculations are based upon density functional theory (DFT), but with varying detailed approximations and trade-offs between computational cost and accuracy.…”

“…The first documented encounter can be traced back to the original work of Seebeck on thermoelectric current generation on different materials and alloy pairs in 1819-1827 [1,2]. Quantitative measurements of the Seebeck voltage of ZnSb have been carried out by Becquerel in 1866 [3], and thermoelectric generators using Zn-Sb alloy were fabricated for practical purposes since 1870 [4].…”

Evaluation of energy filtering effect from first principles calculations

“…As more and more TE materials are reported, theoretical modeling of energy filtering effect is helpful to further optimize the TE performance of existing materials and to design new TE composites. There are a number of theoretical reports in the literature about energy filtering effect, some of them are proof‐of‐principle studies , and some modeled real materials, to name a few, PbTe , Sb 2 Te 3 , ZnSb , Mg 2 Si , etc. In these real material modeling, the band dispersions are usually generated by fitting parameters from experiments, therefore, the approach can only be applied to the specific material.…”